The fermentation of carbohydrates to form butanol and acetone by Clostridium acetobutylicum (hereafter abbreviated C. acetobutylicum) was disclosed by Weizmann in U.S. Pat. No. 1,315,585. For many years, this process was used for the preparation of acetone and butanol, and a certain amount of ethyl alcohol was obtained as a by-product.
Eventually, the microbial process was displaced by chemical processes which provide the same products using cheap fossil fuel raw materials. However, the gradual depletion of petroleum fossil fuel with the resultant increase in prices of petrochemical feedstocks has revived interest in the fermentation reaction that uses carbohydrates, which are renewable raw materials.
One problem encountered in the earlier commercial production of butanol by the fermentation process was the long time before the microorganism began to produce appreciable quantities of butanol. These processes typically used a parent culture that was stored as spores. In order to initiate a fermentation, the spores were placed in an aqueous medium containing various nutrients, subjected to a thermal shock and then passed through three growth stages in vessels of increasing size before they were used to inoculate the fermentation tank. This process required about 3 days for inoculum development plus an additional 2 to 3 days to complete the fermentation.
A second problem with batch fermentations was the instability of the microorganism in concentrations of butanol about about 1% by weight. The cells tend to die rapidly after the concentration of the product reaches this level. Thus, only low concentrations of product could be obtained in the fermentation broth. Such a process requires a large fermentation capacity to produce practical amounts of solvent, and it is expensive to recover the solvents from the fermentation broth due to their low concentrations.
Various workers have suggested that if the butanol fermentation were conducted in a continuous process, some of the difficulties associated with the batch mode might be overcome. Such a process would make more efficient use of the fermentors by eliminating the unproductive time between batches when the fermentor has to be emptied, cleaned, filled, sterilized, and cooled. In the past, such processes have been of little usefulness because of culture instability, contamination problems, or low volumetric productivity of butanol.
The term "volumetric productivity" as used in this application refers to the amount of a product produced in a continuous fermentation per unit volume of fermentation broth per unit of time. It is commonly reported in grams per liter-hour, abbreviated g/l-hr.
One report of continuous butanol fermentation studies was given by Dyr, et al, in: Continuous cultivation of Microorganisms (Prague Symposium), pp. 210-226, Prague: Czechoslovakia Academy of Sciences (1958). By means of a multistage fermentation system and by adjusting the dilution rate, he was able to overcome some of the problems of culture instability, but the highest concentration of butanol reported indicated a volumetric productivity of only about 0.24 g/l-hr for the system.
Another method which has been proposed to avoid the culture instability problem associated with continuous fermentations is the use of spores or nongrowing cells of the microorganism immobilized in a gel. Haggstrom, et al, International application PCT/SE80/00231. Although this technique does provide a means for conducting a continuous fermentation for comparatively long times, it uses an expensive material and process for immobilizing the microorganism. The reported volumetric productivity of butanol by this method is about 0.33 g/l-hr.
A process has now been discovered which avoids the problems inherent in the batch process and which permits the fermentation to be carried out in a continuous process for a number of days without culture deterioration. In addition, this process can be carried out using raw materials readily available from the corn wet-milling industry, and it produces butanol at a much greater volumetric productivity. Furthermore, it provides as an integral part of the process an economical means for the concentration and recovery of the butanol produced in the fermentation.